diff --git a/taglib/mpeg/id3v2/id3v2frame.cpp b/taglib/mpeg/id3v2/id3v2frame.cpp
index ae9e6ef3..d60ce4a7 100644
--- a/taglib/mpeg/id3v2/id3v2frame.cpp
+++ b/taglib/mpeg/id3v2/id3v2frame.cpp
@@ -57,17 +57,20 @@ public:
   Frame::Header *header;
 };
 
-bool isValidFrameID(const ByteVector &frameID)
+namespace
 {
-  if(frameID.size() != 4) {
-    return false;
-  }
-  for(ByteVector::ConstIterator it = frameID.begin(); it != frameID.end(); it++) {
-    if( (*it < 'A' || *it > 'Z') && (*it < '1' || *it > '9') ) {
+  bool isValidFrameID(const ByteVector &frameID)
+  {
+    if(frameID.size() != 4)
       return false;
+
+    for(ByteVector::ConstIterator it = frameID.begin(); it != frameID.end(); it++) {
+      if( (*it < 'A' || *it > 'Z') && (*it < '1' || *it > '9') ) {
+	return false;
+      }
     }
+    return true;
   }
-  return true;
 }
 
 ////////////////////////////////////////////////////////////////////////////////